Transcript 100_Thin_films

Electron probe
microanalysis
EPMA
Thin Film Analysis
What’s the point?
EPMA is traditionally done for bulk material.
What are the issues for thin films?
How precise/accurate are such analyses?
Bulk vs thin film
Normal EPMA assumes that the electron beam is exciting a
homogeneous volume, i.e. there is no difference either
laterally or vertically. Thus, the matrix correction is being
applied in a uniform matter, and there is one applicable f(rz)
profile for each element .
As research has improved the accuracy of the f(rz) profiles,
it is now possible to take thin films (including multiple films)
and apply f(rz) models and calculate best fits for unknown
parameters. For example, if you know that there is a TiO2 skin
atop your Ti metal, you can acquire Ti and O X-ray counts at
several E0 values, and then try to match them by modeling
various film thicknesses with 3rd party software programs. Or
if you have been able to measure a film thickness, you could
use EPMA to determine what the phase stoichometry is.
MC Simulation: TiO2 on Ti
It can be helpful to run Monte Carlo simulations
of thin films. Here, the new Casino software is
used to model 15 keV electrons hitting a 1 mm
layer of TiO2 on Ti. Red trajectories to top left
are BSEs. At bottom left is a model of the O Ka
f(rz) profile (blue), plus the profile of X-rays
predicted to escape (red) and get to the detector.
Here the range of
various energy incident
electrons are modeled.
Thin Film Software-1
Thin films can be studied with the electron
microprobe, although the acquired data
cannot be run through (matrix corrected
by) the normal probe software — which
only works for homogeneous volumes.
How it’s done: With the probe, acquire xray intensities on both standards and
unknown at various accelerating
voltages (minimally 3, preferably more;
e.g. 5, 10, 15 keV). K-ratios are then
calculated, and then plotted up against the
accelerating voltage.
Then the thin film program — here the
costly STRATAGem (~$6K)— is run
either in forward or inverse mode. In
forward mode, a film composition and
thickness (and substrate) are input, and the
program then uses the physical mode (phirho-Z) to determine what the k-ratios
should be at various voltages, and you then
try to match your experimental data for the
best fit. In reverse mode, you input all your
experimental data and the program tries to
come up with a best fit solution.
STRATAGem also (importantly) takes
secondary fluorescence into account.
Thin Film Software-2
Another option (cheaper) the
freeware GMRFilm (written
by R.Waldo of the GM epma
lab). STRATA-Gem is very
slick and has a Windows
interface, whereas GMRFilm
runs under DOS and requires
manual tabulating.
In this example,we are trying to determine the effect of a 0.1 mm oxide coat (TIO2)
on the surface of Ti metal. The program proceeds in steps: it first calculates the
effective x-ray intensity (K-ratio) coming from the top layer (layer 1): it shows
that there will Ti Ka x-ray intensity (as a K-ratio) of 0.073 (we’ll ignore the
Oxygen). And then it estimates that the substrate will yield a Ti K-ratio of 0.864.
So how do we compare the probe data with this? The probe only measures TOTAL
Ti x-rays, so we need to ADD together the 2 Ti K-ratios, which yields 0.937. This
says then that this oxide film on top of Ti ‘robs’ the metal of ~6% of the Ti Ka
counts it should yield. GMRFilm also takes secondary fluorescence into account.
Thin Film Software-3
We have previously introduced the CASINO
Monte Carlo simulation software.
Besides showing the trajectories of electrons,
CASINO also calculates f(rz) curves — x-ray
generation and emission versus depth. These
are quantitative, and the intensities are
integrated and numerical values are shown
next to each colored symbol: you want to look
at the RED curve, confusingly marked
“absorbed intensity”, which is really the
emitted x-ray intensity, and the blue is what is
absorbed by the matrix. There is a value
shown: you can use this as the x-ray intensity
of the unknown sample (thin film on a
substrate).
One more step must be done: you need to set up a simulation on material that will act as the
standard: it could be anything, it could be the same material you really used as a standard for
EPMA. Say you are looking at TiO2, it makes sense to use a bulk simulation of TiO2. And treat
this the same way as STRATAGem or GMRFilm.
Oxygen on Boron metal (2 standards)
This and the next slide
demonstrate the utility
of thin film software.
We needed to verify
that our boron standard
was pure, but there was
a small peak at O ka. I
ran it at 2, 3, 7 & 15
keV (red and black
symbols), and then
tested various
interpretations of the
data. Oxygen as bulk
did not fit, whereas a
12Å oxide film did.
Experiment
Models
Not bulk, but ~12 Å film B2O3 (2 different Boron standards)
Thin film modeled with GMRfilm
Carbon on Boron metal
Not bulk, but ~12-18 Å C film (2 different Boron standards)
Thin film modeled with GMRfilm